Heteropoly-acids such as molybdophosphoric acid and molybdovanadophosphoric acid are known as catalysts used for catalytic gas phase oxidative reactions of methacrolein. However, said heteropoly-acids are thermally unstable and liable to deteriorate. Although an alkali metal or the like can be added to a heteropoly-acid to form an alkali metal salt which has improved thermal resistance, the alkali metal salt exhibits only insufficient catalytic activity and its use results in inferior redox efficiency since catalytic activity for methacrolein oxidation rigidly derives only from free heteropoly-acid.